Processing parameters which affect product quality and productivity in the molding of plastics can be divided into three categories: machine parameters, materials properties, and final product quality specifications. Without some way to monitor materials properties in real-time during the molding operation, the processor must rely solely on controlling machine parameters in order to maintain product quality. Experience has shown that this approach is difficult to undertake because of the nonlinear multi-parameter relationship between the product quality specifications and machine variables. A more effective strategy is real-time measurement and control of materials properties because their relationships to final product behavior are less complicated and often based on well founded physical laws. However, carrying out materials measurements in real-time is more difficult than adjusting machine parameters because the technology heretofore available for in-line, in-situ materials measurements has been inadequate.
For the polymer injection molder, crystallization and shrinkage of production parts are important materials properties. For example, the measurement of crystallization growth in real-time could be used to determine the time of product solidification, knowledge of which could in turn be used to minimize mold cycle time. Part shrinkage is one of the most important pieces of information for a molder because the primary purpose of molding is to form a polymer into the specific geometry of specified dimensions, and the final dimensions of the product are equal to the shape of the mold cavity minus shrinkage. For crystallizable polymers, the amount of shrinkage is of more concern than for a glass forming polymer because of the relatively large contraction upon crystallization. The amount of crystallization not only determines the magnitude of contraction and shrinkage of the product but also its mechanical properties. Thus, dimensional and mechanical properties consistency of the product can be maintained by measuring and controlling crystallization. Another approach to maintaining dimensional stability is to measure and control the rate of shrinkage at a specified time in the mold cycle by feedback control of operating conditions such as temperature of injected resin, temperature of the mold, injection pressure and hold pressure.
The concept of using optical measurements as an in-situ, real-time tool to monitor injection molding has been disclosed in Bur et al., Proc. Soc. Plastics Eng. Ann. Tech. Mtg., May 1993, p. 135, which describes an injection molding machine optically instrumented using optical fibers to transmit light from the mold cavity. The described method, however, uses fluorescence spectroscopy as an indicator of the onset of polymer solidification in the mold and requires the use of fluorescent dyes doped into the resin prior to processing. The obligatory presence of such fluorescent dyes can affect the optical properties of the polymer material and of the finished molded article. Thus, there is a need for a monitoring method which does not require the use of a fluorescent dye.
The use of light scattering by semi-crystalline polymeric materials as a tool to characterize the basic materials properties of crystalline polymers has been reported in the literature. Light transmission and scattering by crystallizable polymer materials have been described by Stein et al. who observed that scattering during the crystallization of polyethylene showed an increase in scattered light intensity which passed through a maximum and then decreased [See J. Polym. Sci., Vol. 20, p. 327 (1956); J. Polym. Sci., Vol. 45, p. 521 (1960); J. Appl. Phys., Vol. 31, p. 1873 (1960)] and that light transmitted through crystallizing polyethylene passed through a minimum [See J. Polym. Sci., Pt. C, No. 18, p. 1 (1967)]. The physical basis for the maximum in scattered light intensity (or minimum in transmitted light) has been attributed to the difference in index of refraction of a growing crystal spherulite and its surrounding amorphous medium [See Yoon et al., J. Polym. Sci. Polym. Phys., Vol. 12, p. 735 (1974)]. However, no attempt was made to use this knowledge to detect either the onset of crystallization or shrinkage of a polymeric resin during molding.
Chrisman et al., U.S. Pat. No. 4,672,218 describes a method for optically determining the onset of crystallization of dissolved solids from a solvent or mother liquor which utilizes an optical fiber probe containing illuminating fibers and collecting fibers immersed in a vessel containing the solution of crystallizable material. Although this method purports to detect the onset of crystal nucleation, it is not suitable for detecting the onset of crystallization of a molded polymer. The method relies on reflection and backscattering of light for detection of crystallization, and it is well known that in polymer crystallization backscattering or reflection from growing crystals first occurs when the crystal size is many times the wavelength of the incident light or when there is multiple scattering or reflection from closely packed microcrystals. Typical microcrystal size of polymers grown under conditions of large supercooling is only 20 to 1000 nm [See Peterlin, Encyclopedia of Polymer Science and Engineering, Vol. 10, page 26, H. F. Mark et al. eds., John Wiley, New York (1986)]. This size is too small to backscatter light except when the microcrystals are closely packed. Thus, in polymer crystallization the method of this patent would not detect initial nucleation and crystal growth, and it could only be used to monitor resins of high crystallinity with closely packed microcrystals, a situation which occurs near the completion of crystallization.
Despite the efforts of the prior art, there has remained a need for a method and apparatus for monitoring crystallization and shrinkage of polymeric material during molding.